CAUSES OF WEATHERING 265 



290. Causes of Weathering. The chief agencies of 

 weathering have already been named (cf. 289) ; they are 

 air, water, plants, and animals. Of these, air and water 

 are the most important. Both air and water produce (1) 

 physical changes (cf. 98) in rock, by wearing off and 

 carrying away rock material; and (2) chemical changes, 

 in that they actually alter the nature of the minerals of 

 the rock. Thus the carbon dioxide and water of the air 



Fig. 228. 



Sand Dune Being Carried by the Wind into the Grand Calumet River at 

 Miller's, Ind. Negative by Geo. D. Fuller. 



(carbonic acid; cf. 126) act chemically in decomposing 

 feldspar (cf. 285), in converting limestone into a soluble 

 compound (cf. 132), and in "dissolving" the cementing 

 material of many rocks, thus causing the rocks to crumble 

 Air can also act chemically upon rocks by oxidizing them 

 as it does iron (cf. 48). 



We see the air (wind) acting as a mechanical, or physical, agent in 

 carrying off dust, and in moving the sand of a sandbank, or dune, and 



